Programming and erasing structure for a floating gate memory cell and method of making

ABSTRACT

A floating gate memory cell has a floating gate in which there are two floating gate layers. The top layer is etched to provide a contour in the top layer while leaving the lower layer unchanged. The control gate follows the contour of the floating gate to increase capacitance therebetween. The two layers of the floating gate can be polysilicon separated by a very thin etch stop layer. This etch stop layer is thick enough to provide an etch stop during a polysilicon etch but preferably thin enough to be electrically transparent. Electrons are able to easily move between the two layers. Thus the etch of the top layer does not extend into the lower layer but the first and second layer have the electrical effect for the purposes of a floating gate of being a continuous conductive layer.

RELATED APPLICATIONS

Patent application Ser. No. 10/944,239, titled “Programming and ErasingStructure for a Floating Gate Memory Cell and Method of Making,”assigned to the assignee hereof, and filed concurrently herewith.

FIELD OF THE INVENTION

This invention relates to semiconductor devices, and more particularly,to semiconductor device structures useful for floating gate memorycells.

RELATED ART

One of the desirable characteristics of a floating gate memory is tohave a relatively large amount of capacitive coupling between thecontrol gate and the floating gate. This results in more voltagetransfer to the floating gate from the control gate during program anderase for a given control gate bias. Thus more floating gate to controlgate capacitance results in requiring lower voltages for achievingprogram and erase and/or improving the speed of programming and erase.Another desirable characteristic is to have low variation in thisfloating gate to control gate capacitance. If this capacitance variesthen the range of threshold voltage increases for the erased state. Awider distribution of erased threshold voltages can result in difficultyin reading erased bits and programming erased bits due to leakage. Onetechnique for increasing the control gate to floating gate capacitancewhile minimizing its variation uses an etch into the floating gate overthe isolation region. This does provide some increase in capacitance,but further improvement would still be desirable. A disadvantage of thisapproach is that it is difficult to scale as the technology scales tosmaller dimensions.

Thus, there is a need for device structures that improve programming anderasing and/or reduce voltages for programming and erasing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitedby the accompanying figure, in which like references indicate similarelements, and in which:

FIG. 1 is a cross section of a device structure at a stage in processinguseful in producing the device structure according to a first embodimentof invention;

FIG. 2 is a cross section of the device structure of FIG. 1 at a stagein processing subsequent to that shown in FIG. 1;

FIG. 3 is a cross section of the device structure of FIG. 2 at a stagein processing subsequent to that shown in FIG. 2;

FIG. 4 is a cross section of the device structure of FIG. 3 at a stagein processing subsequent to that shown in FIG. 3;

FIG. 5 is a cross section of the device structure of FIG. 4 at a stagein processing subsequent to that shown in FIG. 4;

FIG. 6 is a cross section of the device structure of FIG. 5 at a stagein processing subsequent to that shown in FIG. 5;

FIG. 7 is a cross section of the device structure of FIG. 6 at a stagein processing subsequent to that shown in FIG. 6;

FIG. 8 is a cross section of the device structure of FIG. 7 at a stagein processing subsequent to that shown in FIG. 7;

FIG. 9 is a cross section of a device structure as a second embodimentof the invention at a stage in processing;

FIG. 10 is a cross section of the device structure of FIG. 9 at a stagein processing subsequent to that shown in FIG. 9;

FIG. 11 is a cross section of the device structure of FIG. 10 at a stagein processing subsequent to that shown in FIG. 10;

FIG. 12 is a cross section of the device structure of FIG. 11 at a stagein processing subsequent to that shown in FIG. 11;

FIG. 13 is a cross section of the device structure of FIG. 10 at a stagein processing subsequent to that shown in FIG. 10 and as a thirdembodiment of the invention; and

FIG. 14 is a cross section of the device structure of FIG. 13 at a stagein processing subsequent to that shown in FIG. 13;

FIG. 15 is a cross section of a device structure as a fourth embodimentof the invention at a stage in processing;

FIG. 16 is a cross section of the device structure of FIG. 15 at a stagein processing subsequent to that shown in FIG. 15;

FIG. 17 is a cross section of the device structure of FIG. 16 at a stagein processing subsequent to that shown in FIG. 16;

FIG. 18 is a cross section of the device structure of FIG. 17 at a stagein processing subsequent to that shown in FIG. 17;

FIG. 19 is a cross section of the device structure of FIG. 18 at a stagein processing subsequent to that shown in FIG. 18;

FIG. 20 is a cross section of the device structure of FIG. 19 at a stagein processing subsequent to that shown in FIG. 19;

FIG. 21 is a cross section of the device structure of FIG. 20 at a stagein processing subsequent to that shown in FIG. 20;

FIG. 22 is a cross section of the device structure of FIG. 21 at a stagein processing subsequent to that shown in FIG. 21;

FIG. 23 is a cross section of a device structure as a fifth embodimentof the invention at a stage in processing;

FIG. 24 is a cross section of the device structure of FIG. 23 at a stagein processing subsequent to that shown in FIG. 23;

FIG. 25 is a cross section of the device structure of FIG. 24 at a stagein processing subsequent to that shown in FIG. 24;

FIG. 26 is a cross section of the device structure of FIG. 25 at a stagein processing subsequent to that shown in FIG. 25;

FIG. 27 is a cross section of the device structure of FIG. 26 at a stagein processing subsequent to that shown in FIG. 26;

FIG. 28 is a cross section of the device structure of FIG. 27 at a stagein processing subsequent to that shown in FIG. 27;

FIG. 29 is a cross section of the device structure of FIG. 28 at a stagein processing subsequent to that shown in FIG. 28;

FIG. 30 is a cross section of a device structure as a sixth embodimentof the invention at a stage in processing;

FIG. 31 is a cross section of the device structure of FIG. 30 at a stagein processing subsequent to that shown in FIG. 30;

FIG. 32 is a cross section of the device structure of FIG. 31 at a stagein processing subsequent to that shown in FIG. 31; and

FIG. 33 is a cross section of the device structure of FIG. 32 at a stagein processing subsequent to that shown in FIG. 32.

Skilled artisans appreciate that elements in the figure are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help improve theunderstanding of the embodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In one aspect, a floating gate memory cell has a floating gate in whichthere are two floating gate layers. The top layer is etched to provide acontour in the top layer while leaving the lower layer unchanged. Thecontrol gate follows the contour of the floating gate to increasecapacitance between the control gate and the floating gate. The twolayers of the floating gate can be polysilicon separated by a very thinetch stop layer. This etch stop layer is thick enough to provide an etchstop during a polysilicon etch but thin enough to be electricallycoupled. Because the etch stop layer is thin, electrons are able to movebetween the two layers. Thus the etch of the top layer does not extendinto the lower layer but the first and second layer have the electricaleffect for the purposes of a floating gate of being a continuousconductive layer. This is better understood by reference to the FIGs.and the following description.

Shown in FIG. 1 is a device structure 10 comprising a substrate 12, anisolation region 14, a tunnel dielectric 16, and a floating gate layer18, preferably of polysilicon. This is a conventional structure exceptthe relative thickness of floating gate layer 18 is less than for aconventional floating gate layer of polysilicon. The thickness in thisexample is preferably about 500 Angstroms, which is less than for aconventional floating gate layer. Substrate 12 is preferably a bulksilicon substrate but could be an SOI substrate and/or a differentsemiconductor material than just silicon. Isolation region 14 is madeusing the common technique of forming a trench and filling it with adielectric, preferably oxide, but could be another dielectric materialsuch as nitride.

Shown in FIG. 2 is device structure 10 after formation of an etch stoplayer 20, which is preferably oxide, on floating gate layer 18. Layer 20is preferably about 15 Angstroms of grown oxide. Layer 20 can also bedeposited and can be another material, for example nitride.

Shown in FIG. 3 is device structure 10 after formation of a floatinggate layer 22 preferably of silicon. This is preferably about the samethickness as floating gate layer 18.

Shown in FIG. 4 is device structure 10 after formation of a patternedphotoresist layer 26 that has openings 30 and 32 over isolation region14. These openings 30 and 32 are on opposing sides of isolation region14.

Shown in FIG. 5 is device structure 10 after an etch of layers 22, 20,and 18 through openings 30 and 32. This etch is first through layer 22that is preferably polysilicon. A chemistry change then etches throughlayer 20. Then the etch chemistry is switched back to that used foretching through layer 22. This extends openings 30 and 32 throughfloating gate layers 18 and 22 and etch stop layer 20.

Shown in FIG. 6 is device structure 10 after formation of a patternedphotoresist layer 33 having an opening 34 between openings 30 and 32.Opening 34 is substantially centered between isolation regions 14 inthis cross sectional view.

Shown in FIG. 7 is device structure 10 after an etch through opening 34through layer 22 that stops on layer 20. A change in etch chemistryresults in opening 34 extending also through layer 20. If it isdesirable to define opening 34 as a sub-lithographic feature, theninstead of using patterned photoresist 33, a hard mask of, for example,nitride could be patterned and then have the opening partially filledwith a sidewall spacer. The technique of partially filling an openingwith sidewall spacers to make the opening smaller is well understood inthe art.

Shown in FIG. 8 is device structure 10 after formation of a dielectriclayer 36 and a control gate layer 38. Dielectric layer 36 is preferablya conventional three layer dielectric oxide-nitride-oxide layer that isabout 125 to 150 Angstroms in thickness. Dielectric layer 36 functionsas an interlevel dielectric between the control gate and the floatinggate of the floating gage memory cell. Control gate 38 is preferably apolysilicon layer that is about 1000 to 2000 Angstroms thick. Afterformation of this control gate layer 38, it is etched to form an actualcontrol gate. This etch to form an actual control gate does not changethe cross section shown in this FIG. 8. Thus, the resulting structure ofFIG. 8 is the final structure of a floating transistor useful as afloating gate memory cell that is also a non-volatile memory cell.

This device structure shows that control gate 38 has an increase in thesurface area adjacent to floating gate layer 22 in opening 34 whileretaining the surface area adjoining floating gate layers 18 and 22 inopenings 30 and 32. The sidewalls of this remaining portion of layer 22provide for not only increased capacitance but also very repeatableincreased capacitance. The thickness of layer 22 is relatively easy tocontrol. Layer 20 is very thin so that electrons pass across thatstructure quite easily. This oxide is not of the same quality as anoxide that is used as a gate dielectric so the electrons are notblocked, especially at the programming and erasing voltages that arecommonly used. Even oxides of the highest quality have leakage at 15Angstroms. Thus with the higher voltages than are used in transistorshaving the thin gate oxides there is substantial electron flow throughthe oxide. Thus, the electrons that are accumulated in floating gatelayer 22 during a programming operation can freely reach floating gatelayer 18 and for these purposes, layer 20 can be considered electricallytransparent. As a further enhancement, more than one opening such asopening 34 can be performed in the process between FIG. 5 and FIG. 6.More openings such as opening 34 are beneficial because that wouldfurther increase the control gate to floating gate layer capacitance.

Shown in FIG. 9 is a device structure 50 comprising a substrate 52analogous to that of substrate 12, a trench 54 analogous to trench 14, atunnel dielectric 58 analogous to that of tunnel dielectric 16, and afloating gate layer 60 analogous to floating gate layer 18 on tunneldielectric 58. FIG. 9 depicts a conventional structure except thatfloating gate layer 60 is not as thick as for the conventionalstructures. Floating gate layer 60 is preferably about 500 Angstroms inthickness. In device structure 50, isolation region 54 surrounds andextends above floating gate layer 60.

Shown in FIG. 10 is device structure 50 after formation of etch stoplayer 62 analogous to etch stop layer 20 on floating gate layer 60 andfloating gate layer 64 analogous to floating gate layer 22 on etch stoplayer 62. Etch stop layer 62 is shown on the sidewall of isolationregion 54 and extending over isolation region 54. When grown as anoxide, etch stop layer is unlikely to be visible on trench 54 but isshown for the case where etch stop layer 62 is deposited, such asdeposited nitride.

Shown in FIG. 11 is device structure 50 after formation of patternedphotoresist portion 66 and an etch of floating gate layer 64 usingpatterned photoresist portion 66 as a mask. This leaves openings 67 and68 between the remaining portion of floating gate layer 64 and trench54. In the process from FIG. 10 to FIG. 11, all of layer 64 is removedexcept that protected by photoresist 66.

Shown in FIG. 12 is device structure 50 after removal of the portion ofetch stop layer 62 not under patterned photoresist portion 66, removalof patterned photoresist portion 66, formation of dielectric layer 69analogous to layer 36 on floating gate layer 60, over the remainingportion of layer 64, and on the exposed portion of trench 54, andformation of control gate layer 70 analogous to control gate layer 38 ondielectric layer 69. A subsequent etch through layers 70, 69, 64, 62,and 60 result in the formation of a floating gate memory cell structureanalogous to that for the structure of FIG. 8. Thus, the devicestructure of FIG. 12 is the resulting device structure for the memorycell. This shows that control gate layer 70 has increased surfaceadjacent to floating gate layer 64 due to the etch of layer 64 prior tothe formation of control gate layer 70. The sidewalls of this remainingportion of layer 64 provide for not only increased capacitance but alsovery repeatable increased capacitance. The thickness of layer 64 isrelatively easy to control.

Shown in FIG. 13 is an alternative device structure 71 formed after FIG.10 to that of device structure 50 of FIG. 11. Device structure 71differs from device structure 50 of FIG. 11 by formation of sidewallspacers 72 and 73 from floating gate layer 64 on the sidewalls ofisolation region 54 by performing an anisotropic etch on floating gatelayer 64 of FIG. 10. Sidewall spacers 72 and 73 are formed inherentlyduring the etch of layer 64. In the case of device structure 71 of FIG.13, this etch of layer 64 is stopped after the etch has reached layer 62but before these sidewall spacers 72 and 73 have been removed. In thecase of device structure 50 of FIG. 11, this etch continues so that thesidewall spacers are removed.

Shown in FIG. 14 is a device structure 71 after formation of dielectriclayer 76 analogous to dielectric layer 36 on sidewall spacers 72 and 73and the remaining portion of floating gate layer 64 and control gatelayer 74 analogous to control gate layer 38 on dielectric layer 76. Inthis cross sectional view FIG. 14 shows a completed memory device. Inthis case, sidewall spacers 72 and 74 become part of the floating gatelayer of the memory device. These sidewall spacers have a largervertical dimension than a horizontal dimension so that their presenceprovides the benefit of an increase in control gate to floating gatelayer capacitance. A potential disadvantage is the difficulty incontrolling the vertical dimension of these sidewall spacers so thatthere may be an increase in the variation in the control gate tofloating gate layer capacitance.

Shown in FIG. 15 is a device structure 100 comprising a substrate 112analogous to substrate 12, an isolation region 114 analogous toisolation region 114, a gate dielectric layer 116 analogous to gatedielectric layer 16, a floating gate layer 118 analogous to floatinggate layer 18, a thin etch stop layer 120 analogous to etch stop layer20, and a sacrificial layer 122 on etch stop layer 20. Sacrificial layer122 may be oxide or nitride or other material but is of a material thancan be etched selective to that of etch stop layer 120. Sacrificiallayer is in this example about the same thickness as floating gate layer118. Other thicknesses may also be effective.

Shown in FIG. 16 is device structure 100 after formation of a patternedphotoresist portion 123 over sacrificial layer 122 and substantiallycentered, in the dimension shown, between isolation regions 114.

Shown in FIG. 17 is device structure 100 after etching sacrificial layer122 using patterned photoresist portion 123 as a mask. This etch isstopped by etch stop layer 120.

Shown in FIG. 18 is a device structure 100 after removal of photoresistportion 124 and formation of a floating gate layer 124 that ispreferably polysilicon, which is conformal. Floating gate layer 124 isabout the same thickness as floating gate layer 118.

Shown in FIG. 19 is device structure 100 after an anisotropic etch thatforms sidewall spacers 126 and 128 from floating gate layer 124 onsidewalls of the remaining portion of sacrificial layer 122.

Shown in FIG. 20 is device structure 100 after removal of the remainingportion of sacrificial layer 122 so that sidewall spacers 126 and 128are free standing on etch stop layer 120. The width of sidewall spacers126 and 128 are largely determined by the thickness of floating gatelayer 124, which can be altered as desired. Floating gate layer 124should be thick enough so that sidewall spacers 126 and 128 can berepeatedly free standing.

Shown in FIG. 21 is device structure 100 after formation of a patternedphotoresist layer that 129 that is opened over isolation regions 114 andan etch through etch stop layer 120, and floating gate layer 118.

Shown in FIG. 22 is device structure 100 after removal of patternedphotoresist layer 129, the formation of a dielectric layer 130 analogousto dielectric layer 36 over etch stop layer 120 and in the openings overthe isolation region 114, and the formation of control gate layer 132analogous to control layer 38 over dielectric layer 130. This crosssection of FIG. 22 depicts a completed floating gate memory cellstructure. Sidewall spacers 126 and 128, which form part of the floatinggate material of the memory cell, thus provide a structure forincreasing the control gate to floating gate layer capacitance whilealso retaining the increased capacitance along the sidewalls of thefloating gate layer in the area over the isolation region. Iflithographic capability allows, additional sidewall spacers similar tosidewall spacers 126 and 128 can be formed by leaving more than oneportion of floating gate layer 123 on etch stop layer 120 betweenisolation regions 114.

Shown in FIG. 23 is a device structure 140 having a substrate 142analogous to substrate 12, an isolation region 144 analogous toisolation region 14, a gate dielectric layer 146 analogous to gatedielectric layer 16, a floating gate layer 148 analogous to floatinggate layer 18, and a sacrificial layer 150 on floating gate layer 148.Sacrificial layer 150 is preferably oxide but could be another materialsuch as nitride. Sacrificial layer 150 is may be etched selective tofloating gate layer 148. In the preferred case of floating gate layer148 being polysilicon, either oxide or nitride are effective assacrificial layer 150. Sacrificial layer is preferably about the samethickness as floating gate layer 148.

Shown in FIG. 24 is device structure 140 after formation of a patternedphotoresist portion 152 and an etch through sacrificial layer 150 usingpatterned photoresist portion 152 as a mask to leave a portion ofsacrificial layer 150 between isolation regions 144. The remainingportion of sacrificial layer 150 is substantially between insideisolation regions 144 in the dimension shown in the cross section ofFIG. 24

Shown in FIG. 25 is device structure 140 after removal of patternedphotoresist portion 152 and formation of a floating gate layer 154 overfloating gate layer 148 and the remaining portion of sacrificial layer150. In the preferred case of floating gate layer 154 being polysilicon,floating gate layer 154 is conformal so that the portion of floatinggate layer 154 is higher over the remaining portion of sacrificial layer150.

Shown in FIG. 26 is device structure 140 after achemical-mechanical-polishing (CMP) process has been applied to leavefloating gate layer 154 with a top surface even with that of theremaining portion of sacrificial layer 150. The CMP process performed inthis way results in the top surface of the remaining portion ofsacrificial layer 150 being exposed.

Shown in FIG. 27 is device structure 140 after the remaining portion ofsacrificial layer 150 has been removed. This leaves an opening 155 infloating gate layer 154. Opening 155 is substantially centered betweenisolation regions 144 in the direction shown in this cross section.

Shown in FIG. 28 is device structure 140 after formation of a patternedphotoresist layer 156 that has openings over the isolation region 144and an etch in those openings through floating gate layer 154 andfloating gate layer 148.

Shown in FIG. 29 is device structure 140 after formation of dielectriclayer 158 in the openings over isolation region 144 and in opening 155and formation of control gate layer 156 160. Dielectric layer 158 isanalogous to dielectric layer 36. As a cross section, FIG. 29 depicts acompleted memory device having increased control gate to floating gatelayer capacitance as a result of opening 155 while retaining theincreased capacitance in openings over isolation region 144.

Shown in FIG. 30 is a device structure 170 comprising a substrate 172analogous to substrate 12, an isolation region 174 analogous toisolation region 54, a gate dielectric layer 176 analogous to gatedielectric layer 58, a floating gate layer 178 analogous to floatinggate layer 60, a sacrificial layer 180 on floating gate layer 178, and apatterned photoresist portion 182 on sacrificial layer 180. Photoresistportion 182 is between isolation regions 174 and substantially centeredin this cross sectional view. Sacrificial layer 180 is preferably oxidebut could be another material such as nitride. Sacrificial layer 180should be able to be selectively etched with respect to floating gatelayer 178.

Shown in FIG. 31 is device structure 170 after etching sacrificial layer180 using photoresist portion 182 as a mask and formation of a floatinggate layer 184 over isolation region 174, floating gate layer 178, andthe remaining portion of sacrificial layer 180. Preferably floating gatelayer 184 is polysilicon which deposits conformally but is shown asbeing planar. This is because the proximity of the sides of theremaining portion of sacrificial layer 180 and isolation region 174 hasthe effect of causing a conformally deposited layer to appear planar. Ifthe proximity is greater then layer floating gate layer 184 may appearconformal, which is not a problem so long as it is sufficiently thick.It is preferably that it be as thick as the height of the remainingportion of sacrificial layer 180.

Shown in FIG. 32 is device structure 170 after a CMP process has beenapplied. This results in a relatively planar surface so that the heightof floating gate layer 184 is the same as the height of the remainingportion of sacrificial layer 180 and is even with the top surface ofisolation region 174. This results in the top surface of the remainingportion of sacrificial layer 180 being exposed.

Shown in FIG. 33 is device structure 170 after removing the remainingportion of sacrificial layer 180 to form an opening 186, formation of adielectric layer 188 analogous to dielectric layer 36 in opening 186 andon floating gate layer 184, and formation of a control gate layer 190analogous to control gate layer 38 on dielectric layer 188. A completememory cell is depicted in this cross sectional view. The memory cellhas increased capacitance due to control gate 190 being in opening 186.

The sidewalls of opening 186 of FIG. 33 and 155 of FIG. 29 aresubstantially vertical which is better for increasing capacitance usingthe opening. If the slope of the sidewalls of opening 186 and 155 causedopening 186 and 155 to be larger at the top than at the bottom, thenthere would be less added capacitance due to filling opening 186 and 155with control gate.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the present invention as set forthin the claims below. For example, certain materials and thicknesses weredescribed and these may be varied. Further, in many cases the number offeatures that were created to provide increased capacitance could beincreased inside the isolation regions. Accordingly, the specificationand figures are to be regarded in an illustrative rather than arestrictive sense, and all such modifications are intended to beincluded within the scope of present invention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

1. A floating gate device a substrate, comprising: a substrate; a gatedielectric overlying the substrate; a floating gate having a firstfloating gate portion and a second floating gate portion; wherein: thefirst floating gate portion overlies the gate dielectric, wherein thefirst floating gate portion is continuous; the second floating gateportion overlies the first floating gate portion, wherein the secondfloating gate portion includes a plurality of separate portions over thefirst floating gate portion; an etch stop layer between each of theplurality of separate portions of the second floating gate portion andthe first floating gate portion that permits passage of charge duringprogramming and erasing of the floating gate device; an interleveldielectric overlying the separate portions of the second floating gateportion and the first floating gate portion; and a control gateoverlying the interlevel dielectric, wherein the control gate extendsdown in a region between the separate portions and over the firstfloating gate portion.
 2. A floating gate device of claim 1, wherein thesubstrate further comprises isolation regions, and wherein the pluralityof separate portions of the second floating gate portion are betweenisolation regions and the region between the separate portions isbetween the isolation regions.
 3. A floating gate device of claim 2,wherein at least one of the plurality of separate portions of the secondfloating gate portion is adjacent a sidewall of an isolation region. 4.A floating gate device of claim 1, wherein the etch stop layer issubstantially electrically transparent between each of the plurality ofseparate portions of the second floating gate portion and the firstfloating gate portion.
 5. A floating gate device of claim 1, wherein theetch stop layer has a thickness in a range of approximately 15 to 20Angstroms.